Haven't indulged in AB2 much, but I'm about to and have some basic questions.

It is generally advised to increase driver current capability (no surprise there), but I find no data. Where do you find the grid impedance curve in that region? How do you know if a CF is enough or you need a 3A source follower?

Is it trial-and-error or just ancient Chinese secrets passed from father to son?

Ancient secret. Will be revealed by year's end. But there's a few datasheets that are nice enough to spill the beans.

If you can pump 10 mA comfortably, you can drive 6L6GC (a really excellent tube for this service) well into AB2. I've used MOSFET source followers (IRF820) for this- if you want to be cute, the source followers don't have to run class A...

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"Pity, wrath, heroism, filled them, but the power of putting two and two together was annihilated."- EM Forster

No, you want followers to get AB2 with any kind of acceptable distortion. It could also possibly be done with a transformer at much greater expense and somewhat lower performance; ditto using drive from the plate of a power tube. Followers are easy, help prevent blocking, and can have impeccable performance. There have been some commercial designs (e.g., Fisher 50A) that used transformer-coupled followers and performed quite well.

__________________
"Pity, wrath, heroism, filled them, but the power of putting two and two together was annihilated."- EM Forster